In physics and mechanics, torque is rotational force, or the force necessary to cause an object to rotate around an axis. In most cases, more torque is needed to start the rotational motion than is needed to keep it going once it has begun. This initial force is called breakaway torque.
The amount of breakaway torque needed to move something is determined in part by static friction. Static friction is the force that exists between two physical bodies to keep them from moving. For example, a wing-nut may have high breakaway torque because there is a lot of static friction between the nut and the bolt. If, however, the bolt is greased, the torque will be lower because the static friction has been decreased.
Breakaway torque is almost always discussed in one of two contexts, either the power of an engine or the force needed to turn a threaded fastener such as a nut. If a mechanic wants to remove a nut from a bolt, he or she must apply torque to the nut using a wrench. As anyone who has ever done this knows, turning the wrench takes a lot of force at first, but usually becomes easier after a few turns.
Mechanics and engineers often measure the breakaway torque of threaded fasteners in part to ensure product integrity as part of a process called torque auditing. If the breakaway force of a nut is too low, the vibration of the equipment may cause the nut to loosen; if it's too high, the threads may strip and the bolt may be impossible to remove. Part of testing the fastener includes measuring the torque at the moment the fastener starts moving, after the torque has been overcome. Auditing can either be done using sensors to measure the torque or by a trained operator applying torque by hand.
Another area where breakaway torque is important is in cylinder engines. As with threaded fasteners, the breakaway torque of an engine is greater than the running torque. In an engine, torque is used to spin a crankshaft. The crankshaft in turn causes the pistons to move up and down.
An engine that is designed with only enough power to keep the engine running will fail. The engine must also have enough power to get the movement started in the first place. Yet it is a delicate balance. If the engine is allowed to continue running with enough torque to cause breakaway, it may overheat. Once the breakaway has been achieved, the engine torque should be reduced to a normal operating speed.
How To Calculate Breakaway Torque
Calculating breakaway torque correctly depends on each different situation. First, there are different factors to consider for actions that a human produces versus the ones that a machine does. There are also many similarities. In general, figuring out breakaway torque relies on figuring out all the different forces at play on an object to change it from being at rest to mobile. These forces include both the force the person or machine exerts and the friction forces that work against it.
The General Formulas
The simplest version of the equation that determines force and friction is F=ma where F stands for force, m stands for mass, and a stands for acceleration. When multiple different forces are involved, such as in breakaway torque situations, this formula becomes more complicated. One needs to figure out each different force that exists on an object at any one time and then apply a force greater than the sum of all those forces together to break the object away and cause it to rotate.
Torque is a special kind of rotational force that requires specific modifications to this equation. To calculate torque forces, use the equation T= rFsinθ, where T stands for torque, r stands for radius (usually the length of the lever used), F stands for force, and θ stands for the angle between the force being applied and the radius or lever length. Generally, torque is measured in N-m or newton-meters.
Ways To Lower the Required Initial Torque Necessary
Once a person figures out the minimum amount of force needed to cause rotational movement in a static object, that person has discovered the breakaway torque. Some additional factors can then help to achieve breakaway torque. Lengthening the leveraging object can help achieve a greater downward force to encourage an object to start moving. For a person, this leverage might be a wrench, while for a machine, it is usually the crankshaft and pistons in the engine.
One can also increase the height at which the lever starts, thereby increasing the angle and the force applied to help create greater overall torque. As the above equation for calculating torque indicates, increasing this angle multiplies its effects into the equation and increases the ending amount of torque. In breakaway torque, this helps apply more torque force without having to increase the weight, distance, or mass. Basically, think about this as a form of increasing the acceleration from the first formula, F=ma.
Lubricating the object is another way to help achieve the breakaway torque required. Lubrication causes the frictional forces to decrease. Lowering the forces of friction in turn lowers the amount of force needed to initiate the rotation of an object. Some of the more common lubrication substances are oil and grease.
Another way to affect the friction forces is to use a rough cloth or pad to grip the object to help move it. The uneven surfaces on the object apply additional friction to the object and can help even out or lower the effects of the frictional forces present in the stationary object. This method is especially useful for human-applied torque but less practical for machine-applied torque.
What Is a Breakaway Torque Converter?
A breakaway torque converter is a specially designed part in a machine engine that helps create the needed breakaway torque forces. A torque converter is an additional way to help apply the necessary force to move a stationary object. This device is generally a donut-shaped fluid coupling in combustible engines that helps transfer the rotational forces or power from the engine to the load it is trying to move. One common example is found in automobiles. Automobile torque converters exist between the engine and the transmission specifically in automatic transmission vehicles.